Linear aromatic/heterocyclic condensation polyimides have been an item of commerce for almost two decades. The polymer systems that have seen the most commercial success include those usually described as the reaction products of aromatic tetraacids or derivatives with aromatic diamines. The acids are typically employed in their dianhydride or diester form. Because of the difficulty in shaping, coating or other processing of the polyimides produced from the above starting materials, it has been found to be more convenient to first form a solution of an intermediate reactant from the starting materials. This intermediate is termed a polyamic acid. The polyimide is then formed by concurrent solvent removal and thermal imidization which typically occurs at from about 150.C. to about 300.C. A representative general reaction employed to form the linear condensation polyimides is shown below. ##STR1##
It is most cost effective to use high solids polyamic acid solutions in this process. Further, it is best to employ high molecular weight polyamic acids since the resultant polyimide products have improved properties. It has been previously known that high molecular weight polyimides can be made from low viscosity, high solids solutions of polyamic acids by carboxylating, "end-capping," the terminal amino groups of the polyamic acid with dicarboxylated monoanhydrides. See for example U.S. 4,742,153. Thermal conversion of the end-capped polyamic acids converts these materials to polyimides through formation of an intermediate anhydride and subsequent chain extension of the end-capped polyamic acid. However, end-capped polyamic acids prepared by this process require high cure temperatures and long cure times.